There’s not much more to it. Supposedly during his stay in Washington during World War II as a British diplomat, Roald Dahl (yes that one) decided one night to paint the “undercarriages” of the bison on Dumbarton Bridge. Apparently he chose red.
It’s apparently only an unverified rumor, though. But it shows up in at least two Dahl biographies. One source suggests that it was covered in the gossip columns of the time. GM searched the archives of the Washington Star and the Post but couldn’t find a mention. It’s probably better that way.